Liquid crystal display device and driving method therefor

ABSTRACT

A liquid crystal active matrix display device includes a first substrate having a thin film transistor formed thereon. A light-screening film 1 is located on the first substrate in a manner to be overlapped with an ITO. The display device includes a second substrate having a color filter and a light-screening film 2 formed thereon. The second substrate is opposed to the first substrate. The light-screening film 2 extends from the uncontrollable area by a certain value depending on a voltage from the external, resulting in improving a numerical aperture.

BACKGROUND OF THE INVENTION

The present invention relates to a liquid crystal display device andmore particularly to the structure and the driving method of the displaydevice which provides a high-quality display.

The conventional liquid crystal display device is constructed to have anactive matrix substrate and a light-screening mask formed thereon sothat the display device can provide a high-contrast panel as disclosedin JP-A-1-297623. This construction, however, has a shortcoming that theapplication of the construction to, in particular, a high-definitionliquid crystal display device results in lowering a contrast ratio.

As disclosed in JP-A-2-10955, a storage capacitance element may beprovided in parallel to a liquid crystal unit so as to suppressreduction of stored charges resulting from leakage current of a thinfilm transistor or a liquid crystal unit and diminish variation of aneffective voltage. However, this storage capacitance element also hassome difficulty in working the form of each element located on thesubstrate to be uniform on the plane of the substrate if the liquidcrystal device is required to be more definitive and larger in size,resulting in disadvantageously making the dimensions of the elementsprovided on the substrate variable. The variation leaves an after imageon the display device and degrades the liquid crystal.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide ahigh-definition liquid crystal display device which is capable ofdiminishing leakage of light for realizing a high-contrast display andmaking a numerical aperture larger for providing a brighter display.

It is still another object of the present invention to provide ahigh-definition liquid crystal display device having structure of astorage capacitance element which is designed to diminish variation of astorage capacitance element located on a common potential electronicplane, the occurrence of short-circuit between wires, and a water markfor the purpose of improving a yield.

It is another object of the present invention to provide a drivingmethod of the storage capacitance elements located on the liquid crystaldisplay device which method is suitable to the reduction of variationsof a common electric potential and a threshold value.

In order to achieve the foregoing objects, the present invention isconstructed to provide an opaque material on one substrate having a thinfilm transistor formed thereon and an opaque material on anothersubstrate having a color filter formed thereon so that those opaquematerials serve to screen light applied from an area uncontrolled inresponse to an external voltage (signal voltage, scan voltage, etc.).The opaque materials keep a predetermined distance from theuncontrollable area. Further, in a TFT (thin film transistor) drivingsystem liquid crystal device having two or more pixels located thereonincluding a first wire served as a scan signal line, a field-effecttransistor (FET) whose gate electrode is connected to the first wire, asecond wire served as a signal line and being connected to one of adrain and a source electrodes of the FET, a storage capacitance elementone electrode of which is connected to the other of the drain and thesource electrodes of the FET, a liquid crystal element one electrode ofwhich is connected to the other of the drain and the source electrodesof the FET, a third electrode connected to the other electrode of thestorage capacitance element, and a fourth electrode connected to theother electrode of the liquid crystal element and to the thirdelectrode, the TFT driving system liquid crystal device is characterizedin that a peripheral length μm of a capacitance section of the storagecapacitance element is 1.33 time as large as or lower than a valueobtained by dividing an area μm² of the capacitance section by adiagonal length of an area where the pixels are located and the thirdand the fourth electrodes are connected to a common electrode signalline. The capacitance section of the storage capacitance element hasvertical and horizontal end portions which are terminated only on theend portions of either one of the electrodes of the storage capacitanceelement.

As described above, the peripheral length μm of the capacitance sectionof the storage capacitance element is 1.33 or less times as large as avalue obtained by dividing an area μm² of the capacitance section by adiagonal length of an area where the pixels are located included in thedisplay unit of the TFT driving system liquid crystal display device. Itresults in suppressing variation of areas of the storage capacitanceelements, that is, variation of capacitance values resulting from thedimensional variation to a ±20% value of a central value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing construction of a liquid crystaldisplay system according to an embodiment of the present invention;

FIG. 2 is a circuit diagram showing construction of a liquid crystaldisplay system according to an embodiment of the present invention;

FIG. 3 is a diagram showing an embodiment of a liquid crystal displaymodule;

FIG. 4 is a diagram showing an embodiment of each substrate included inthe liquid crystal display according to the invention;

FIG. 5A is a schematic view showing a plane and a section of each of thepixels composing a liquid crystal panel according to the invention;

FIG. 5B is a schematic view showing a section of each of the pixelscomposing a liquid crystal panel according to the invention;

FIG. 6 is a graph showing a voltage-optical characteristic of twistednematic (TN) liquid crystal;

FIG. 7 is a schematic view showing another embodiment of each pixelincluded in a liquid crystal panel;

FIG. 8 is a view showing another embodiment of a liquid crystal panelaccording to the present invention;

FIG. 9 is a section view of the embodiment shown in FIG. 8 on the lineA-A' of FIG. 8;

FIG. 10 is a section view of the embodiment shown in FIG. 8 on the lineB-B' of FIG. 8;

FIG. 11 is a section view of the embodiment shown in FIG. 8 on the lineC-C' of FIG. 8;

FIG. 12 is a view showing a liquid crystal panel according to anotherembodiment of the present invention;

FIG. 13 is a view showing a liquid crystal panel according to anotherembodiment of the present invention;

FIG. 14 is a view showing a liquid crystal panel according to anotherembodiment of the present invention;

FIGS. 15A and 15B are views showing a liquid crystal display unitincluded in the liquid display device according to the presentinvention;

FIG. 16 is a plan view showing a manufacture process of the liquidcrystal display device according to the present invention;

FIG. 17 is a view showing one pixel included in the liquid crystaldisplay unit of the liquid crystal display device according to thepresent invention;

FIG. 18 is a view showing an essential portion of the liquid crystaldisplay unit of the liquid crystal display device according to thepresent invention;

FIG. 19 is a view showing a color filter pattern included in the liquidcrystal display unit according to the present invention;

FIG. 20 is a view showing a pixel pattern and a color filter patternincluded in the liquid crystal display unit;

FIG. 21 is a view showing an embodiment of a storage capacitance elementincluded in the present invention;

FIG. 22 is a graph showing relation between a capacitance value of thestorage capacitance element and an optimal common electric potential;

FIG. 23 is a graph showing relation between an area of a capacitanceunit included in the storage capacitance element and a peripheral lengthof the capacitance unit;

FIG. 24 is a view showing another embodiment of the storage capacitanceelement unit included in the present invention;

FIG. 25 is a view showing another embodiment of the storage capacitanceelement;

FIG. 26 is a chart showing driving signal waveforms used in anembodiment of the present invention;

FIG. 27 is a chart showing driving signal waveforms used in theconventional liquid crystal display device;

FIG. 28 is a chart showing driving signal waveforms used in anotherembodiment of the present invention;

FIG. 29 is a chart showing an example of a display signal used in thepresent invention;

FIG. 30 is a chart showing driving signal waveforms used in anotherembodiment of the present invention;

FIG. 31 is a view showing another embodiment of the storage capacitanceelement used in the present invention;

FIG. 32 is an explanatory view showing another embodiment of the presentinvention;

FIGS. 33A and 33B are views showing a device to which the liquid crystaldisplay according to the present invention is applied; and

FIGS. 34A and 34B are views showing how a voltage changes on a displayelectrode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Herein, an embodiment of the invention will be described with referenceto the drawings. FIG. 1 shows construction of a liquid crystal displaysystem according to an embodiment of the present invention. The liquidcrystal display system is arranged to have a display system 200 and aninformation processing system 220 such as a work station, a personalcomputer, and a wordprocessor device. The display system 200 includes aliquid crystal display module 202, a light source 201, a light sourceadjusting circuit 203, an image data generating circuit 204A, a timingsignal generating circuit 204B, a liquid-crystal brightness and contrastadjusting circuit 240, a storage capacitance driving voltage generatingcircuit 205, and a common electrode driving voltage generating circuit206.

The liquid crystal display module 202 is arranged to have a liquidcrystal panel 217, a signal circuit 207 for generating a signal voltage,and a scan circuit 208 for generating a scanning voltage.

The liquid crystal panel 217 is arranged to have a thin film transistor102 composed of a-Si, p-Si or the like, a storage capacitance element310, a liquid crystal 57, a signal line 100 for driving the thin filmtransistor 102, and a scan line 101. Each one electrode of the storagecapacitance element 310 and the liquid crystal 57 is connected to one ofa drain/source electrode of the thin film transistor 102. The otherelectrode of the storage capacitance element 310 is connected to astorage capacitance line 106 and the other electrode of the liquidcrystal 57 is connected to a common electrode terminal 213.

The storage capacitance driving voltage generating circuit 205 serves togenerate a Vstg voltage and the common electrode voltage generatingcircuit 206 serves to generate a Vcom voltage. The Vstg voltage isapplied to the storage capacitance line 106 and the Vcom voltage isapplied to the common electrode terminal 213. Those voltage are notspecific and may be at the same voltage level or phase. The other of thedrain/source electrode of the thin film transistor 102 is connected tothe signal line 100.

The light source adjusting circuit 203 may be operated in synchronous tothe liquid-crystal brightness and contrast adjusting circuit 240 and theadjusting method is not specific.

Any connection may be employed as a connection between the storagecapacitance element 310 and the storage capacitance line 106 withouthaving to limit the connection shown in FIG. 2. The connection betweenthe signal line 100 and the signal circuit 207 may be the connectionshown in FIG. 2 where the signal lines are alternately pulled out in thevertical manner and are also alternately connected to the signalcircuits 207A and 207B. However, the connection is not specific to theconnection shown in FIG. 2. No specific connection is prepared for theconnection between the scanning line 101 and the scan circuit 208,though the connection therebetween is not particularly illustrated inFIG. 2.

In FIG. 2, the signal circuit 207 and the scan circuit 208 are, in partor in all, may be formed integrally with the liquid crystal panel. Theresulting device has simpler construction and higher reliability inconnections or the like, hence, is advantageous in lowering the cost. Inthis case, the arranging means of the signal circuit and the scancircuit may take each of;

(1) Means for arranging the circuits on the liquid crystal panel 217 inthe form of the thin film transistor composed of a-Si or p-Si,

(2) Means for mounting a monocrystalline Si substrate having thecircuits formed thereon to the liquid crystal panel 217, and

(3) Means for combining said means (1) and (2).

FIG. 3 shows an embodiment of a liquid crystal display module 202. Theliquid crystal display module 202 is arranged to have a liquid crystalpanel 217, signal circuit substrates 227 to 234, scan circuit substrates222 to 224, voltage-pulling substrates 225, 226, 235, 236 for pullingthe common electrode voltage Vcom and the storage capacitance voltageVstg, and a signal common substrate 220.

The image data generating circuit 204 supplies an image data signal anda supply voltage to the signal common substrate 220 through a signalcable 221.

FIG. 4 shows an embodiment of the signal circuit substrates 227 to 234and the scan circuit substrates 222 to 224. Each of the circuitsubstrates is composed by mounting an integration circuit 237A havingthe signal circuit or the scan circuit formed thereon onto an organicfilm having pattern wires implemented thereon. The pattern wires 237Bserve as output terminals for a scan voltage or a signal voltage and thepattern wires 237C serve as input terminals for the image data signal orthe supply voltage.

The common electrode voltage Vcom is applied to the common electrodeterminals 238A to 238D (FIG. 3) and the storage capacitance voltage Vstgis applied to the storage capacitance line 106 (FIG. 3).

In addition, it is better to employ an elastic substrate made of anorganic film as the voltage-pulling substrates 225, 226, 235, 236 forimplementing more convenient mounting.

FIG. 5A is a plan view schematically showing each of the pixelscomposing the liquid crystal panel 217 and FIG. 5B is a section viewschematically showing the same. The thin film transistor 102, the scanline 101 and the signal line 100 are not illustrated in FIG. 5. Withreference to FIGS. 5A and 5B, the gist of the present invention will bedescribed.

One substrate 61 includes opaque materials 51, 52 for screening avisible ray of light applied from a backlight source 64, an insulatinglayer 55, and a display electrode (transparent) 54 served as a displaypixel formed on a transparent base made of glass, for example. Thedisplay electrode 54 is driven by the thin film transistor 102, thoughit is not illustrated.

The other substrate 62 is located in opposition to the substrate 61.Both of the substrates 61 and 62 keep a gap of about 5 to 10 μmtherebetween. This substrate 62 includes an opaque material 59 forscreening a visible ray of light, a color filter 60 for colorrepresentation, and a common electrode (transparent) 63 formed on atransparent base made of glass, for example.

57 is a liquid crystal, which serves to control the quantity of lightapplied from the backlight source 64 depending on a voltage differencebetween the display electrode 54 and the common electrode 63.

It is preferable to employ as the liquid crystal 57 the twisted nematicliquid crystal (referred to as TNLC) where the nematic liquid crystal istwisted 90° front and back. In addition, a polymer dispersed liquidcrystal (referred to as PDLC) may be used as the liquid crystal 57. Thatis, any liquid crystal and orientation method may be used.

FIG. 6 shows a voltage-optical characteristic of the liquid crystal 57employing the TNLC. This type of display mode is referred to as normalwhite mode, at which the brightness of each pixel becomes smaller as thevoltage V_(LCD) applied to the liquid crystal increases.

To display an image with several tones on a TV or the like, the voltageapplied to the liquid crystal is controlled to be about V_(ON) toV_(OFF) depending on the image signal so as to control the brightness ofeach pixel to be B_(ON) to B_(OFF), resulting in realizing colordisplay. The significant factor for defining the display quality is acontrast ratio (B_(ON) /B_(OFF)). To obtain a high-quality color image,it is essential to enhance the contrast ratio (B_(ON) /B_(OFF)). For thepurpose, it is necessary to sufficiently lower the brightness B_(OFF)corresponding to the applied voltage V_(OFF). Further, to lower thepower consumption of the back light source 64, which is one of thecomponents of the display, for the purpose of conserving the powerconsumption of the overall system, it is necessary to enhancetransmittance of each pixel. On the other hand, in case the liquidcrystal matrix display is applied to an information terminal device withso many pixels such as a CAD or CG display or a HDTV, the liquid crystaldevice has difficulty in meeting the foregoing necessary conditions ofthe liquid crystal panel.

The embodiment shown in FIG. 5 is arranged to meet the foregoingnecessary conditions for realizing a high-quality liquid crystal colordisplay.

As the concrete means for the purpose, the opaque materials 51, 52, 59are provided for screening light from the area uncontrolled by thedisplay electrode 54.

To expand the description more concretely, as shown in FIG. 5A, thedisplay electrode 54 and the opaque materials 51, 52 are overlappedunder the condition of W₂ >0 and W₁ ≧0 (W₁ +W₂ =W_(B)) and the end ofthe opaque material 59 formed on the other substrate 62 is located offthe light-uncontrolled area of the substrate 61 so as to keep a constantdistance (=W_(B)) therebetween.

The opaque material 59 located near the points 53A to 53D where theupper and the lower ends of the opaque materials 51 and 52 are crossedwith the display electrode 54 is formed in a manner to depict a fan witha radius of about W_(B). By defining the dimension of the opaquematerial 51, it is possible to prevent the leakage of the light appliedfrom the backlight source even if the fitting between both of thesubstrates 61 and 62 is slipped vertically and horizontally, resultingin implementing a high-contrast display.

W₁ is equal to or more than the slippage caused in forming the displayelectrode 54 and the opaque materials 51, 52 and W_(B) (=W₁ +W₂) is setto such a value as disallowing the light applied from the backlightsource 64 to be visually seen from the vertically or horizontallyoblique point of view.

FIG. 7 shows another embodiment of the present invention. In theembodiment, the distance between the points 53A to 53D and the end ofthe opaque material 59 is set as ≧W_(B). It results in making the end ofthe opaque material 59 linear, thereby improving the working accuracy.

The opaque material formed on the substrate 61, though it is notillustrative, may be in the same layer or another layer if the conditionshown in FIG. 5A (W₁ ≧0 and W₂ >0) is met. Nothing specifies thelocation of the opaque material.

Further, nothing specifies the number and the form of the opaquematerial as well.

Assuming that the width of minimizing W_(B) (=W₁ +W₂) is W_(B) (min) inthe opaque material meeting the condition shown in FIG. 5A (W₁ ≧0 andW₂ >0), the distance between the area uncontrolled by light on thesubstrate 61 and the end of the opaque material 59 formed on the othersubstrate 62 may be set to be equal to or more than the W_(B) (min).

FIG. 8 shows a liquid crystal panel according to another embodiment ofthe present invention. 102 denotes a transistor composed of a drainelectrode 102A, a source electrode 102B, a gate electrode 102C, and asemiconductor layer 102D. 103 denotes another transistor composed of adrain electrode 103A, a source electrode 103B, a gate electrode 103C,and a semiconductor layer 103D. The semiconductor layers 102D, 103D arepreferably an a-Si type thin film transistor and a p-Si type thin filmtransistor. However, nothing specifies those semiconductor layers 102Dand 103D.

100A and 100B denote signal lines. The signal lines 100A and 100B areelectrically connected to drain electrodes 102A and 103A of the thinfilm transistors 102 and 103 so that a voltage for controlling thebrightness of the liquid crystal is applied to the signal lines 100A and100B. 101A and 101B denote scan lines to which a voltage for switchingthe thin film transistors 102 and 103 on and off is applied.

The display electrode 54 is a transparent electrode made of ITO (IndiumTix Oxide) and is electrically connected to a source electrode 102B. 51and 52 denote opaque materials, which are preferably made of opaquemetal such as Cr or Al. Any material may be used for the opaquematerials 51, 52 if it has a characteristic of screening a visible rayof light.

106 denotes a storage capacitance line which is partially overlappedwith the display electrode 54. The overlapped portion corresponds to thestorage capacitance element 310. 107 denotes a short-circuit electrodeof the display electrode 54. The short-circuit electrode 107 serves toprevent electric separation of the display electrode 54 in the verticalmanner when the display electrode 54 gets over the display electrode 54.

The foregoing components are formed on one substrate. The othersubstrate having a color filter formed thereon is located in oppositionto the substrate. Both of the substrates keep a distance of about 10 μm.The liquid crystal is sealed between both of the substrates.

FIGS. 9, 10 and 11 show sectional structure of the liquid crystal panelshown in FIG. 8 on the 9A-9A', the 10B-10B' and the 11C-11C'. The samecomponents shown in FIGS. 8 to 11 are referenced by the same numbers.

In FIG. 9, 56 denotes a transparent substrate made of glass, forexample. 114 and 55 denote insulating layers formed by SiNx, forexample. 57 denotes a liquid crystal. 58 denotes a transparent substrate(the other substrate) made of glass, for example, 59 denotes an opaquematerial made of opaque metal or an organic material containing blackpigment, 60 denotes a R, G and B color filter, and 63 denotes a commonelectrode formed by a transparent material such as ITO.

As shown in the sectional views of FIGS. 9 to 11, the gate electrodes102C, 103C, the opaque materials 51, 52 and the storage capacitance line106 are formed on the same layer. Another embodiment of the presentinvention will be shown in FIGS. 12 to 14. FIGS. 12 and 13 show theembodiment where the storage capacitance line 106 is shifted to the thinfilm transistor 103. This embodiment makes it possible to provide anexcellent display without dividing each pixel into two parts.

FIG. 14 shows the embodiment where the storage capacitance line 106 isformed integrally with the opaque material so as to increase the storagecapacitance. This embodiment thus makes it possible to provide ahigh-quality display with no unevenness.

In turn, FIG. 15A shows a liquid crystal display section included in anactive matrix color liquid crystal display device. FIG. 15B is asectional view of the 15BB-15BB' of FIG. 15A. FIG. 15B shows thestructure containing the liquid crystal device as well. FIG. 16 is aplan view showing part of a pixel for describing the manufacturingprocess. In FIG. 15B, one pixel included in the liquid crystal sectionis composed of the thin film transistor 102 and the display electrode 54formed on the lower transparent glass base 56, which has a thickness ofabout 1.1 mm, for example. The thin film transistor 102 contained ineach pixel is arranged to mainly have a gate electrode 102c, a gateinsulating film 55, an i-type (no conductive impurity is doped)noncrystalline silicon semiconductor 102D, and a pair of sourceelectrode 102B and drain electrode 102A. Each pixel is located within acrossing area of the adjacent two scan lines 101 and the adjacent twosignal lines 100. Two or more scan lines 101 extend in a columndirection and range in a row direction. Two or more signal lines 100extend in a row direction and range in a column direction.

In turn, the description will be directed to the manufacturing processof the liquid crystal panel.

The liquid crystal 57 is sealed within the space formed between thelower transparent glass substrate 56 and the upper transparent glasssubstrate 58 as being defined by a lower orientation film 418 and anupper orientation film 419 which serve to define the molecularorientation of the liquid crystal. The lower orientation film 418 isformed on a protective film 114 made of silicon nitride which is formedon the lower transparent glass substrate 56. The upper transparent glasssubstrate includes a color filter 60, an organic protective film 452, acommon electrode 63, and an upper orientation film 419 laminated on theinside thereof in the describing sequence. The common electrode 63 isopposed to the display electrode 54 included in each pixel on the sideof the lower transparent glass substrate 56 and is formed integrallywith the upper transparent glass substrate 58. FIG. 15A is a plan viewshowing the overlap of the pixel with the color filter shown in FIG. 16,in which a common voltage V_(com) is applied to the common electrode 63.

The color filter 60 includes a dye carrier formed of a resin materialsuch as acrylic resin. The color filter 60 is located in opposition toeach pixel and between the adjacent two display signal lines 100 in amanner to bridge between the pixels. For each pixel, the correspondingcolor filter 60 is colored. This color filter 60 is formed in accordancewith the following process. At first, a dye material is formed on thesurface of the upper transparent glass substrate 58. All the dye carrierexcept the red filter area is removed by a photolithograph technique.Then, the dye carrier is colored with a red dye and the fixing processis carried out so as to fix the red dye on the dye carrier. Likewise,the green filter and the blue filter are formed. The organic protectivefilm 452 is provided to prevent the dye materials colored to the colorfilter 60 from being leaked into the liquid crystal. The organicprotective film 452 is formed of a transparent resin material such asacrylic resin or epoxy resin.

The lower transparent glass substrate 56 and the upper transparent glasssubstrate 58 are independently formed as described above. Then, both ofthe glass substrates 56 and 58 are combined with the liquid crystalbeing sealed therebetween, resulting in completing the liquid crystaldisplay device.

The central portion shown in FIG. 15B shows one pixel in section. Theleft portion shows the section of the portion where an external pull-outwire exists in the left side edge portions of the transparent glasssubstrates 56 and 58. The right portion shows the section of the portionwhere no external pull-out wire exists in the right side edge portionsof the transparent glass substrates 56 and 58.

460 denotes a sealing member which is shown in the right and the leftsides shown in FIG. 15B. The sealing member 460 is formed on theperipheral edges of the transparent glass substrates 56 and 58 so as toallow the liquid crystal 57 to be sealed within the space defined by theboth transparent glass substrates and the sealing member 460. Thissealing member 460 may be made of epoxy resin, for example.

The common electrode 63 of the upper transparent glass substrate 58 is,at least at one portion, connected to an external pull-out wire formedon the lower transparent glass substrate 56 by means of a silver paste435. With respect to the gate electrode 102C, the source electrode 102Band the drain electrode 102A, the external pul-out wire is formed by thesame manufacturing process.

The orientation films 418, 419, the display electrode 54 and the commonelectrode 63 are formed inside of the sealing member 460. 430 and 431denote polarizing plates, which are respectively formed on the outsidesurfaces of the lower transparent glass substrate 400 and the uppertransparent glass substrate 58.

The gate electrode 102C is formed of an aluminium film and has a filmthickness of about 100 nm. The gate electrode 102C is formed to be largeenough to completely cover the silicon semiconductor layer 102D (ifviewed from the lower side). In case, hence, a backlight source like afluorescent lamp is mounted on the lower side of the lower transparentglass substrate 56, that opaque gate electrode 102C serves as anobstacle to the light applied from the backlight source. Hence, thelight is prevented from being hit on the silicon semiconductor layer102D. It results in lowering a possibility that a conduction phenomenondue to the light emission, that is, the off characteristic degradationof the thin film transistor 102 takes place.

Considering the gate electrode 102C in light of the function of the gateand the light screening, the gate electrode 102C and its wire may beintegrally formed on a single layer. In this case, it is possible toselect aluminium containing silicon, pure aluminium, or aluminiumcontaining palladium as an opaque conductive material.

The gate insulating film 55 of the thin film transistor 102 is formed onthe gate electrode 102C and the upper layer of the scan line 101. Thegate insulating film 55 is made from silicon nitride film by means ofthe plasma CVD method, for example. The thickness of the gate insulatingfilm 55 is about 300 nm. As another gate insulating film, there isprovided an alumina gate insulating film 416 formed by transforming thealuminium film of the gate electrode into alumina by means of theanodizing method, for example. That is, the gate insulating film has thedual-film structure. This alumina gate insulating film 416 serves aspreventing the short-circuit between the gate electrode 102C and thewiring portion formed on the upper layer such as a metal film used forthe scan line 101 and the drain electrode 102A and the source electrode102B.

52 denotes a light-screening film formed in the same process as theterminal portion formed for pulling the scan line 102 out. Thislight-screening film 52 is formed by a sputtering method and is subjectto patterning by means of the photolithography technique. The silicontype semiconductor layer 102D is formed of an amorphous silicon film ora multi-layered silicon film and has a thickness of about 180 nm. Thissilicon semiconductor layer 102D can be formed as changing thecomponents of a supply gas in the plasma CVD apparatus when the siliconnitride gate insulating film 55 is formed. Besides, it is formed withoutbeing exposed to the outside of the apparatus. An N+ layer 102d with adopant of phosphorus for ohmic contact can be formed in the same processso as to have a thickness of about 40 nm. Then, the resulting lowertransparent glass substrate 56 is pulled out of the plasma CVD apparatusand the silicon semiconductor layer 102D is patterned in an insularmanner by means of the photolithography technique.

Next, the display electrode 54 is made of a transparent conductive film(ITO) formed by the sputtering method so as to have a film thickness of120 to 200 nm. Then, the display electrode 54 is patterned at each pixelby means of the photolithography technique.

The source electrode 102B and the drain electrode 102A are respectivelycomposed by laminating a first conductive film A and a second conductivefilm B on the lower surfaces of those electrodes 102B and 102A which arein contact with the N+ semiconductor layer 102d. The first conductivefilm A contained in the source electrode 102B and the drain electrode102A uses a chrome film formed by the sputtering method and has athickness of 50 to 100 nm. As the chrome film is made thicker than agiven thickness, the stress caused in the chrome film becomes larger.Hence, it is necessary to define the thickness of the chrome film to be200 nm or less. The chrome film keeps in good contact with the N+semiconductor layer 102d so that the chrome film can prevent diffusionof aluminium contained in the second conductive film B (to be describedlater) to the N+ semiconductor layer 102d, that is, the chrome filmserves as a barrier layer. The first conductive film may be formed of ahigh melting point metal film (Mo, Ti, Ta, W) and a high melting pointmetal silicide (MoSi₂, TiSi₂, TaSi₂, WSi₂) in addition to chrome. Thesecond conductive film B is formed of aluminium to have a thickness of300 to 400 nm by means of the sputtering method. Since the aluminiumfilm has smaller stress than the chrome film, the thicker aluminium filmthan the chrome film can be formed so as to diminish the resistancevalues of the source electrode 102B, the drain electrode 102A and thesignal line 100. The second conductive film B is arranged to enhance anoperating speed and a signal transmission speed of a display signal ofthe thin film transistor 102. That is, the second conductive film Benhances a writing characteristic of the pixel. To form the secondconductive film B, it is also possible to use an aluminium filmcontaining silicon (Si) or copper (Cu) as an additive. The sourceelectrode 102B and the drain electrode 102A composed of the firstconductive film A and the second conductive film B are respectivelypatterned by means of the photolithography technique. The N+semiconductor layer 102d is partially removed with the masks of thephotolithomask, the first conductive film A and the second conductivefilm B. That is, the N+ semiconductor layer 102d left on the siliconsemiconductor layer 102D are removed except the portions having thefirst and the second conductive films A and B.

FIG. 16 is a plan view showing the manufacturing process from the startto the foregoing steps.

Then, the silicon nitride is formed on the lower transparent glasssubstrate 56 to have a thickness of 1 μm by means of the plasma CVDmethod. With the photolithography technique, the essential portions suchas a terminal portion are exposed from the silicon nitride film and thena silicon nitride film 114 is covered on the overall pixel forprotecting the pixel.

FIG. 17 shows one pixel of a liquid display portion included in theactive matrix liquid crystal display device according to the embodimentof the present invention and FIG. 18 shows an essential portion of theliquid crystal display portion having two or more pixels locatedtherein.

The scan lines 101 extend in the row direction and range in the columndirection. The signal line 100 extends in the column direction andranges in the row direction. The storage capacitance lines 106 extendbetween the adjacent scan lines 101 in the row direction in parallel tothe scan lines 101 and range in the column direction.

Those signal lines are connected to the corresponding driving circuitslocated around the liquid crystal display portion. That is, each scanline 101 is connected to the terminal portion formed on the transparentglass substrate at the tip end of the scan line 101 extended in the rowdirection, for example, the left end of the scan line 101. Each terminalportion is connected to a TAB within which each terminal portion isconnected to each output portion of the signal driving circuits. Thestorage capacitance line 106 is connected to a common electrode at thetip end of the storage capacitance line 106 extended in the rowdirection, for example, the right end of the storage capacitance line106 and then to the terminal portion. The terminal portion is connectedto an electrode formed on an FPC which leads to the output portion ofthe storage capacitance driving voltage generating circuit 205.

As shown in FIG. 17, the thin film transistor 102 contained in eachpixel is arranged to mainly have a gate electrode (scan line) 102C, aninsulating film, a noncrystalline silicon semiconductor 102D, and a pairof source electrode 102B and drain electrode 102A. It is to beunderstood that the definition of the electrodes as the source and thedrain depends on the bias polarity and the definition is switcheddepending on the reverse operation of the polarity. The followingdescription is, however, expanded on the assumption that the sourceelectrode and the drain electrode are fixed for easier understanding ofthe present invention. In the pixel, the thin film transistor 102 islocated on the scan line 101 located under the pixel. This scan line 101serves as a gate electrode of the thin film transistor 102. The channeldirection (direction of the current flowing between the source and thedrain) of the thin film transistor 102 is located in parallel to thedirection of the signal line 100. The end portion of the thin filmtransistor 102 is connected to the display electrode 54. The drainelectrode 102A is located under the thin film transistor 102 and isconnected to the signal line 100 located on the left side of the pixel.In the present embodiment, hence, the pixel is controlled by the scanline 101 located under the pixel and the signal line 100 located on theleft side of the pixel itself. Further, this embodiment assumes thefactor W/L defining mutual conductance gm, that is, a ratio of a channellength L (distance between the source and the drain electrodes) to achannel width W of the thin film transistor 102 as about 3. This valueis defined in light of dimensional shift in working in addition to aframe frequency, the number of scan lines, movability of a thin filmtransistor, a liquid crystal capacitance value, a storage capacitancevalue, and so forth.

The storage capacitance line 106 is located between the adjacent scansignal lines 101. The substantially uniform interval is kept between thestorage capacitance line 106 and the scan line 101. The square displayelectrode 54 is located within the area defined by the adjacent two scanlines 101 and the adjacent two signal lines 100 and the storagecapacitance element 310 is formed at the cross point of the storagecapacitance line 106 and the display electrode 54. The storagecapacitance element 310 provides a capacitance value per one pixeldefined depending on the W/L of the thin film transistor 102, theoverlapping capacitance (Cgs) between the source electrode 102B and thescan line 101, and so forth. The area of the storage capacitance element310 is defined depending on the capacitance value per a unit area of theinsulating film. In the present embodiment, the storage capacitanceelement 310 is rectangular. The vertical length (as viewed in thevertical direction of FIG. 17) is defined on the fact that thehorizontal length is equal to the length of the display electrode.

A get-over electrode 323 is provided on the display electrode 54. Theget-over electrode 323 is formed in the layer where the source and thedrain electrodes are formed and serves to electrically connect theoverlapped portion of the display electrode 54 with the storagecapacitance line 106 to the other portion of the display electrode 54.It results in making it possible to prevent display failure due todisconnection of the display electrode 54 at a step portion of thestorage capacitance line 106. The liquid crystal element is formed onthe portion where the display electrode 54 is not overlapped with thestorage capacitance line 106.

At the cross points between the signal line 100 and the scan line 101and between the signal line 100 and the storage capacitance line 106,noncrystalline silicon semiconductors 305 and 311 are respectivelyprovided. Those semiconductors 305 and 311 are formed on the layer wherethe noncrystalline silicon semiconductor 102D of thin film transistor102 is formed.

The display electrode 54 occupies a maximum area unless theshort-circuit takes place between the display electrode 54 and thedisplay signal line 100, the noncrystalline silicon semiconductors 305,311, the drain electrode 102A, or the like. The display electrode 54provides the light-screening layers 51, 52 at the end portions thereofso as to partially prevent light from being leaked from the periphery ofthe display electrode 54. The display electrode 54 is at the samepotential as the source electrode 201B. Hence, writing the potential ofthe display signal line 100 in the display electrode 54 and holding thepotential of the display electrode 54 are controlled depending onswitching the thin film transistor 102 on or off.

The pixels, each arranged as shown in FIG. 17, are ranged in the rowdirection and the column direction at each pitch of a horizontaldimension 316 and a vertical dimensional 317 as shown in FIG. 18. Theupper transparent glass substrate is provided in opposition to the lowertransparent glass substrate formed as shown in FIG. 18.

FIG. 19 shows a color filter pattern provided on the upper transparentglass substrate where two or more pixels are located. In FIG. 19, eachpixel frame with the horizontal dimension 316 and the vertical dimension317 is shown by a broken line for clarifying the position relationbetween the pixel pattern on the lower transparent glass substrate andthe color filter pattern. FIG. 19 is a plan view of the color filterpattern viewed from the back surface of the upper transparent glasssubstrate (the opposite side to the liquid-crystal side). As will beclearly appreciated from FIG. 19, the color filter is arranged so as tocorrespond to each pixel and colored. That is, like the pixel, the colorfilter is formed on the area where the adjacent two scan lines arecrossed with the adjacent two signal lines. On the surface of the inside(liquid-crystal side) of the upper transparent glass substrate, severalpatterns are formed including a light-screening layer 108, a red filterlayer (R) 60R, a green filter layer (G) 60G, and a blue filter layer (B)60B. And, the common electrode 63 covers on the overall surface of theliquid crystal display portion. Those patterns extend in the columndirection and range in the row direction in the sequence of red, greenand blue. It means that the filter color is unique along each patterncolumn. That is, the color filter has the structure where verticalstripes are located.

FIG. 20 shows the pixel patterns formed on the lower transparent glasssubstrate and the color filter patterns formed on the upper transparentglass substrate at the same time. In the liquid crystal display deviceaccording to the present invention, the mixture of the R, G and B pixelsranged in parallel results in realizing multi-color display. It meansthree pixels ranged horizontally compose one display unit (1 dot) 322.The vertical dimension of each dot is set substantially equal to thehorizontal dimension. Hence, the horizontal dimension 316 of one pixelis set to be 1/3 time as large as the vertical dimension 317 thereof.

The desired number of dots arranged as above compose the liquid crystaldisplay section. On the back surface (opposite side to the liquidcrystal) of the lower transparent glass substrate of the liquid crystaldisplay section, there is provided a light source (backlight). A voltage(an effective value of an a.c.voltage) is kept between the transparentpixel electrodes of the pixels formed on the lower transparent glasssubstrate and the common transparent electrodes formed on the uppertransparent glass substrate. The voltage is applied to the liquidcrystal sealed between the upper and the lower glass substrates so as tochange the molecular orientation of the liquid crystal, thereby tochange the light transmittance of the light applied from the backlightsource. This change results in realizing the display. To enhance thedefinition of the liquid crystal display device, the dimension of onedot is set smaller. By setting the dimension of one dot to be 0.2 mm orsome, for example, the liquid crystal display device can provide highdefinition.

In turn, the description will be directed to the arrangement of thestorage capacitance element and the driving method therefor. At first,some embodiments will be described about the storage capacitance elementwhich is suitable to reducing variation of the optimal common potentialon the same plane.

FIG. 21 shows an embodiment for describing the storage capacitanceelement used in this invention. The storage capacitance element iscomposed of a crossing portion 310 between the first electrode 106 madeof a storage capacitance line and a second electrode 54 being a displayelectrode opposed to the first electrode 106 through the insulatinglayer. The storage capacitance element 310 includes an upper end 506 anda lower end (as viewed in FIG. 21) both terminated at the end portionsof the first electrode 106 and a left end 504 and a right end 505 bothterminated at the end portions of the second electrode 54.

    L<1.33S/D                                                  (1)

The expression (1) features the arrangement of the storage capacitanceelement 310. In the expression (1), L denotes a total length (μm) ofsides 504, 505, 506, 507 of the capacitance portion, S denotes an area(μm²) of the crossing portion 310, that is, the storage capacitanceelement, and D denotes a diagonal length (inch) of an area in thedisplay section where the pixels are located. The area variation of thecapacitance portion caused when the element dimensions are variable onthe display surface is denoted by LΔX. ΔX denotes a slippage, that is, avariation (μm) of an average dimension of the worked elements. ΔX isnormally proportional to D, though ΔX depends on the working technique.Hence, ΔX=aD is established. According to an experiment implemented bythe inventors, the proportional constant a is about 0.15. Thus,

    ΔX=0.15D                                             (2)

In the 10-inch display device which is often available as a displayterminal for OA equipment, ΔX is about 1.5 μm. A rate of the area of thestorage capacitance element to the capacitance portion caused when thedimensions are variable on the plane, that is, a rate of variation of acapacitance value is denoted by LΔX/S.

The voltage applied on the display electrode 54 depends on the change ofthe scan voltage.

FIG. 34B shows how the display electrode 54 is changed. The voltage ofthe display electrode is made equal to the signal voltage for switchingthe thin film on. Then, when the scan voltage Vg shifts into a lowpotential, the thin film transistor is switched off. At a time, thesource voltage Vs drops by Vp when the scan voltage Vg rises.

To prevent degradation of a characteristic of the liquid crystal 57 andof a response time of the image display, it is preferable that thed.c.components of the voltage caused between the voltage of the sourceelectrode 102B and the common potential Vcom are reduced to 0 V.

Depending on the drop of the source voltage Vx, the common potential isrequired to drop. The common potential at this time is referred to as anoptimal common potential. The change of the source voltage Vs due to thescan voltage Vg is referred to as a coupling voltage Vp.

FIG. 22 shows the relation between the capacitance value of the storagecapacitance element and the optimal common potential. The curve shown inFIG. 22 is true to any pixel used in the normally available liquidcrystal display in addition to the present invention. Assuming that thestorage capacitance is Cstg, the parasitic capacitance between thesource and the gate of the thin film transistor is Cgs, the liquidcrystal capacitance is Clc, and the potential difference between whenthe scan line (gate) is on and when it is off is VgHL, the couplingvoltage can be represented as follows.

    Vp=Cgs/(Cgs+Cstg+Clc)·VgHL                        (3)

When the capacitance value Cstg of the storage capacitance element issmall, the coupling voltage Vp becomes larger based on the equation (3),resulting in lowering the optimal common potential. The presentinventors have found out from the dependency of the optimal commonpotential on the capacitance value of the storage capacitance elementthat the variation of the optimal common potential can be suppressed tobe 200 mV or lower independently of the capacitance value if the ratioof the variation of the capacitance value is about 20% or less. Hence,if the equation of

    LΔX/S<0.2                                            (4)

is established, no failure takes place such as an after image ordegradation of liquid crystal. The equation (1) can be obtained from theequations (2) and (4).

FIG. 23 shows the relation between the capacitance portion area S andthe peripheral length L of the capacitance portion. The curve 522 showsa circular capacitance portion, that is, a minimum peripheral length Lof the capacitance portion. The peripheral length L of the capacitanceportion corresponds to the area located above the curve 522. The line521 shows the equation (1) in which the sign of inequality is replacedwith a sign of equality. The equation (1) indicates the peripherallength of the capacitance portion corresponds to the area located abovethe line 521. That is, the oblique-line area enclosed by the curve 522and the line 521 corresponds to an area in which there exist suchperipheral lengths of the capacitance portion as suppressing thevariation of the optimum common potential to be 200 mV or less. As willbe appreciated from the equation (1), as the diagnosis line D of thedisplay portion is made longer, the line 521 has a smaller gradient,resulting in narrowing the oblique-line area. As will be appreciatedfrom FIG. 28, as the area of the capacitance portion is made smaller,the oblique-line area is made narrower. That is, as the liquid crystaldisplay device provides a larger screen and as the area of thecapacitance portion is made smaller because the screen is reduced insize as a result of making the liquid crystal display device moredefinitive, the peripheral length of the capacitance portion has asmaller allowance area. It results in putting the significance on theform and the dimension of the storage capacitance element. The presentinvention becomes significant in case the diagnosis length of thedisplay portion contained in the liquid crystal display device is 9 inchor more, the diagnosis length of one dot (one display unit consisting ofthree pixels R, G and B) is 400 μm or less or the area of one pixel is30000 μm² or less

According to the curve shown in FIG. 22, it is considered that as thestorage capacitance element provides a larger capacitance value, thevariation of the optimum common potential is made smaller when thecapacitance value changes and the variation of the optimum commonpotential is suppressed to be 200 mV or less even if the variation rateof the capacitance value is about 20% or more. However, herein, it isnecessary to vary the optimum common potential depending on the signalpotential of the signal line. That is, if the storage capacitanceelement provides too small capacitance, the optimum common potentialvaries greatly depending on the signal potential, because the differencebetween jump voltages based on the potentials appearing on the signalwire is too conspicuous. For example, if a small voltage is applied tothe liquid crystal capacitance, the jump voltage becomes larger thanthat if a larger voltage is applied, thereby making the optimum commonpotential smaller. Hence, a d.c.voltage is applied on the other in-planelocation where another signal potential is applied except the in-planelocation where the optimum common potential is properly adjusted. Itresults in bringing about some failures such as an after image anddegradation of liquid crystal. If the storage capacitance elementprovides too large capacitance, the degree of the writing based on thepotential of an image signal line greatly changes. Conversely, if alarge voltage is applied to the liquid crystal capacitance, the writingratio is made lower on a positive polarity side than the writing ratioif a small voltage is applied. It results in varying the optimum commonpotential and bringing about some failures such as an after image ordegradation of the liquid crystal like the above. It is, therefore,necessary to set the capacitance value of the storage capacitanceelement to a proper value. In actual, it is effective that thecapacitance value of the storage capacitance element is three to seventimes as large as the capacitance value of the liquid crystal.

Returning to FIG. 17, the description will be directed to the storagecapacitance element 310. In FIG. 17, one electrode (storage capacitanceline) 106 of the storage capacitance element 310 is substantiallyperpendicular to the other electrode (display electrode) 54 and thestorage capacitance element 310 is substantially rectangular. Theelectrode 106 of the storage capacitance element 310 is made of alow-resistance metal such as Al. The electrode 106 composes the commonelectrode portion and is located in parallel to the gate electrode (scansignal line) 101. The display electrode 54 of the storage capacitanceelement 310 is a pixel electrode which is a transparent conductive filmmade from indium tin oxide (ITO), for example. The display electrode 54includes a light-transmitting area formed outside of the storagecapacitance line 106 through the insulating film. This area becomes anopening portion. Between the storage capacitance line 106 and thedisplay electrode 54 is laminated an insulating film made of a siliconnitride film or a compound film consisting of a silicon nitride film andan Al₂ O₃ film formed by anodizing Al. Such a film is used in anotherembodiment. The horizontal width of the display electrode 54 is designedto have the largest length unless it short-circuits with the adjacentdisplay signal line 100. Hence, since the storage capacitance element isrectangular, the vertical width of the storage capacitance line 106 atthe location corresponding to the storage capacitance element is mademinimum with respect to the set area of the storage capacitance element.At the crossing portion 523 of the storage capacitance line 106 and thesignal line 100, the crossing area is designed to be as small aspossible, because the short-circuit between the storage capacitance line106 and the signal line 100 is likely to take place. Hence, the width ofthe storage capacitance line 106 on the crossing portion 523 is designedto be normally finer than the line width on the capacitance portion 310.In order to reduce a probability of occurrence of short-circuit, on thecrossing portion 523, an amorphous silicon layer 311 is located betweenthe storage capacitance line 106 and the signal line 100. The get-overelectrode 323 is intended to connect the display electrode 54 locatedover the step formed at the end portion of the storage capacitance line106 in order to avoid the disconnection of the display electrode 54resulting from the step difference. In the embodiment, the relationbetween an area and a length of the storage capacitance element 310meets the relation of the equation (1). It results in reducing variationof the capacitance resulting from the dimensional variation of theelements and thereby bringing about no failure such as an after image ordegradation of liquid crystal. Further, since the storage capacitanceelement portion of the storage capacitance line 106 is arranged to havea minimum vertical width, it is possible to keep a sufficiently longinterval between the storage capacitance line 106 and the scan line 101forming a gate electrode located in parallel. It means that there iseven a small possibility of occurrence of the short-circuit between thescan line 101 and the storage capacitance line 106. As will beappreciated from FIG. 17, the upper and lower ends of the storagecapacitance element are terminated at the electrodes and the right andleft ends thereof are terminated at the other electrodes. Further, bothof the electrodes keep the width constant within the distance from theend portion of the crossing portion between the two electrodes to theslipping portion of the electrodes. The slippage hence does not changethe area of the crossing portion, that is, the capacitance portion. As aresult, since no variation of the capacitance value of the storagecapacitance element is brought about based on the slippage from theproper position, no excessive d.c. voltage is partially applied to theliquid crystal provided on the screen, resulting in bringing about nofailure such as an after image or degradation of the liquid crystal.

FIG. 24 shows another embodiment of the present invention. As shown, theillustration is limited to the storage capacitance line 106 and thedisplay electrode 54 composing the storage capacitance element 310.According to the present embodiment, the relation between an area and aperipheral length of the storage capacitance element 310 meets theequation (1). Those electrodes keep their widths constant within thedistance from the crossing portion of the storage capacitance element106 and the display electrode 54 to the slipped portion between at leastthe two layers. Hence, the variation of a capacitance value resultingfrom the dimensional variation is quite small. Since the storagecapacitance element does not bring about any variation of thecapacitance value resulting from the slippage from the proper position,no failure takes place such as an after image or degradation of theliquid crystal. Further, in the present embodiment, the storagecapacitance line 106 does not provide linear upper and lower ends, theend surfaces of which are directed into at least three directions. Sincethe storage capacitance line 106 provides step portions directed in atleast three ways, the display electrode 54 can be sufficiently fitted tothe storage capacitance line 106 at any one of the step portionsdirected in the three ways and the fitting is strong enough to preventthe disconnection. The concave portions 528 and 529 provided in thestorage capacitance element 310 may be convex portions. Further, thoseconcave portions have no limitation in their positions, size and form.In case the relation between the area and the peripheral length meetsthe equation used in the description of FIG. 21 and the upper and thelower ends are directed into at least three ways, the effect of thisembodiment is true to the construction.

FIG. 25 shows another embodiment of the present invention. As shown, thestorage capacitance element 310 provides an upper end 531 and a rightand a left ends 532, 533 terminated at the display electrode 54. In thisembodiment, the relation between an area and a peripheral length of thestorage capacitance element 310 meets the equation used in thedescription of FIG. 21. Hence, the variation of a capacitance valueresulting from the dimensional variation is very small and no failure isbrought about such as an after image and degradation of the liquidcrystal. Since the display electrode 54 is terminated at the upper end531, the display portion is located only under the storage capacitanceelement 310. The arrangement where the storage capacitance element 310does not disconnect the display portion therefore makes it possible toavoid lowering a resolution. Since the display electrode 54 gets overthe step of the storage capacitance line 106 at one portion, thedisconnection of the display electrode 54 is unlikely to take place.This embodiment is also effective in case of the additional capacitanceelement whose electrode is a gate electrode at the front step.

The reduction of an after image or degradation of the liquid crystaldescribed in the foregoing embodiments can be achieved by the othermethod. That is, the parasitic capacitance Cgs between the gate and thesource is 5% or less of a sum of the storage capacitance Cstg and theliquid crystal capacitance Clc. This setting makes it possible to reducethe coupling voltage and the dependency of the optimal common potentialon the capacitance value of the storage capacitance element. Hence, itis not necessary to provide the constraint shown in FIG. 21 to the formof the storage capacitance element.

Then, the description will be directed to a driving method for a storagecapacitance element which is suitable to reduction of the variation ofthe optimal common potential or the threshold value.

FIG. 26 shows the waveforms of driving voltages used in an embodiment ofthe present invention. (a) of FIG. 26 is the waveform of a voltageapplied to the gate electrode (scan line), (b) is the waveform of avoltage applied to the drain electrode (signal line), (c) is thewaveform of a voltage applied to the common electrode signal line, and(d) shows the relative relation among those waveforms, in which thoseoverlapped waveforms are shown by dotted lines and the waveform of avoltage applied to the source electrode is shown by a real line. At atime t1, the gate electrode starts to rise from an OFF voltage VGL to anON voltage VGH, when the writing is started. Hence, the source electrodeis started to change toward the potential of the drain electrode. Thegate electrode reaches the ON voltage VGH at a time t2.

At the time t2, the drain electrode start to shift its voltage to adesired voltage VDH. A certain lag time later, the voltage of the drainelectrode reaches VDH. At a time t3, the common electrode starts toshift its voltage from the voltage VCH to VCL. A lag time tCD later, thevoltage of the common electrode reaches VCL. The writing is continueduntil, at a time t5, the voltage of the gate electrode shifts from theON voltage VGH to the OFF voltage VGL. During the interval, the voltageof the source electrode reaches a desired voltage VDH.

Between the time t4 and the time t5 when the writing is completed, thedriving waveforms of the voltages used in this embodiment are at thepositive polarity. At the time t5, a desired voltage VDH-VCL is appliedto the liquid crystal element and the storage capacitance element. Thepulse width (1/2 of one period) of each voltage of the drain electrodeand the common electrode is identical to the writing time tW (t5-t1 whenone scan signal line is selected). At the time t5, the voltage of thegate electrode starts to drop from the ON voltage VGH. A lag time tGDlater, at a time t6, the voltage of the gate electrode returns to theOFF voltage VGL. During the interval, the voltage of the sourceelectrode drops by just a coupling voltage Vp because of electrostaticinduction. At a time t6, the voltage of the gate electrode remains OFF.When the coupling is terminated, the voltage of the drain electrodestarts to change. At a time t7, a certain time tA later than the timet6, the voltage of the common electrode starts to shift from the voltageVCL to VCH, when the voltage of the source electrode starts to changebecause of the electrostatic induction. A certain time tA later than thetermination of the coupling, the voltage of the source electrode startsto change. Hence, the slippage of timing of the driving waveform of thevoltage of the common electrode has no effect on the driving waveform ofthe voltage of the source electrode. FIG. 27 shows the conventionaldriving waveform. The voltage waveform applied to the drain electrode issynchronous to that applied to the common electrode so that the voltagesof the drain and the common electrodes start to change at a time. Hence,at the time t6, after the jumping operation, the voltage of the sourceelectrode starts to rise. It means that the modulation of the drivingwaveform is likely to effect on the voltage of the source electrode. Thelag time tA of the voltage of the common electrode against the voltageof the drain electrode is zero (for the conventional method) or largerbut smaller than the one scan line selecting time tW minus the lag timetGD of the signal line (drain electrode) against the scan line (gateelectrode) and the maximum lag time tCD of the common electrode. Inactual, the voltage of the common electrode compensates for the couplingvoltage and is set somewhat low for removing the d.c.components of thevoltage applied to the liquid crystal. That is, the voltage of thecommon electrode is adjusted to the optimal common voltage.

FIG. 28 shows the waveform when the lag time tA of the voltage of thecommon electrode against the voltage of the drain electrode is maximum,that is, the one scan signal line selecting time tW minus the lag timetCD of the signal line (drain electrode) against the scan line (gateelectrode) in the embodiment of the present invention. In this case, atthe time t5 when the writing is terminated, the voltage of the commonvoltage Vc reaches a predetermined voltage. Also in this case, thevoltage of the source electrode starts to change a certain time tA laterthan the termination of the coupling. Hence, the slippage of the timingof the driving waveform of the voltage of the common electrode has noeffect on the waveform of the voltage of the source electrode. In thisembodiment, while the voltage is at the positive polarity (after thewriting is terminated), the difference among the voltages of the commonelectrode, the drain electrode and the source electrode is made small.Hence, it is unlikely that the thin film transistor having as a gate thecommon electrode formed on the color filter side glass substrate carriesout the parasitic MOS operation. It results in keeping the holdingcharacteristic stable.

The potential of the drain electrode defines the potential of the sourceelectrode, that is, the transparent pixel electrode for each pixel. Torealize the multi-color display, this potential is divided into severaltones. Each tone voltage is defined depending on how the lighttransmittance of the liquid crystal depends on the voltage applied tothe liquid crystal. The voltage applied to the liquid crystal is defineddepending on the potential difference between the signal line and thecommon electrode line. Since the common electrode line is common to allthe pixels, the change of the potential applied on the signal lineresults in the division of the potential into several tones. FIG. 29shows one example of the division. The potential difference between thesignal line and the common electrode line is divided into eight tones.The voltage applied on the common electrode line defines the potentialsetting of the signal line. In FIG. 29, the fourth or less tones are inthe same phase as the common electrode line. That is, by keeping thepotential of at least one tone to be in the same phase as the commonelectrode line, the difference between the ON voltage of the gate andthe writing voltage is kept constant for any tone, resulting in reducingthe dependency of the jump voltage on the display signal potential.

FIG. 30 shows the driving waveform of the voltages used in anotherembodiment. In FIG. 30, (a) denotes a waveform of a voltage applied atthe gate electrode, (b), (c) and (d) denote waveforms of the voltagesused in the conventional driving method. Concretely, (b) denotes awaveform of a voltage applied on the drain electrode if the luminance onthe overall surface is constant, (c) denotes a waveform of a voltageapplied to the common electrode, and (d) denotes a waveform of a voltageapplied to the drain electrode if the luminance on the surface isvariable. (e), (f) and (g) denote the waveforms of voltages used in thedriving method according to the present invention. Concretely, (e)denotes a waveform of a voltage applied on the drain electrode when theluminance on the overall surface is constant, (f) denotes a waveform ofa voltage applied on the common electrode, and (g) denotes a waveform ofa voltage applied on the drain electrode when the luminance on thesurface is variable. In the present embodiment, the waveforms of (e),(f) and (g) are twice as large as the pulse width (selecting time forone scan line) on the common electrode. That is, the conventionalwriting sequence is a positive polarity, a negative polarity, a positivepolarity, and a negative polarity for each scan line, while the presentwriting sequence is a positive polarity, a positive polarity, a negativepolarity, and a negative polarity for each scan line. It results inhalving the driving frequency and thereby conserving the powerconsumption of the driving circuit. As shown in (e) of FIG. 30, thepresent driving frequency of the voltage applied on the drain electrodeis half as large as the conventional driving frequency if the luminanceon the overall surface is constant, while in actual, the present drivingfrequency is as large as the conventional driving frequency, because theluminance on the surface is variable. In case of the high-definitionliquid crystal device, the tone difference between the adjacent pixelsis small. Hence, as shown in (g) of FIG. 30, the driving frequency issubstantially reduced in half, resulting in making it possible toconserve the power consumption of the driving circuit. The pulse widthof the voltage of the common electrode is n times as large as thewaveform pulse width (selecting time for one scan line) of the voltageapplied on the gate electrode. n denotes a divisor of all the scanlines. According to the present embodiment, n is 2. It may be possibleto combine the method of the present embodiment with theasynchronization of the potential of the common electrode.

Next, the direction will be directed to the arrangement of the storagecapacitance element which is suitable to reduction of the short-circuitbetween the wires and the watermark and improvement of a yield.

FIG. 31 shows an embodiment of the present invention. As shown, thestorage capacitance element 310 is ranged horizontally at a pixel pitch.The storage capacitance line 106 extends horizontally so as to composeone electrode of the storage capacitance element 310 contained in eachpixel. 543 denotes a connecting portion between the pixels of thestorage capacitance line 106. The connecting portion 543 is made moreslender than the electrode portion of the storage capacitance element310 and is connected at the vertically central position of the right andthe left ends of the storage capacitance element 310. The storagecapacitance line 106 and the scan line 101 are alternately ranged at thesubstantially same intervals. The connecting portion 543 of the storagecapacitance line 106 is located at the vertically central portion of theright and the left ends of the storage capacitance element. 544 and 545denote valley portions resulting from the step of the storagecapacitance line 106. The valley portions 544, 545 are, on the average,made shorter because of the foregoing location of the connecting portion543. Hence, in the process for manufacturing the thin film transistor,there is a low probability that the liquid for washing and etching staysin the concave portion, which results in causing failure. The locatingmethod of the storage capacitance element according to the inventionmakes it possible to reduce the short-circuit between the wires and thewater mark, thereby improving the yield.

As shown by the oblique lines of FIG. 31, the storage capacitanceelement 310 may be located on the upper or the lower side of theadjacent pixel. The upper end and the lower end of the connectingportion 543 of the storage capacitance line 106 match to the upper endof the storage capacitance element 310 and the lower end of the storagecapacitance element 310'. Hence, the connecting portion 543 provides novalley portion resulting from the step of the storage capacitance line106 so that no etching liquid is substantially allowed to stay in theprocess of manufacturing the thin film transistor, resulting in makingit difficult to cause failure. The locating method of the storagecapacitance element according to the present invention serves to reducethe watermark and improve a yield. In the present embodiment, thestorage capacitance elements are located in such a manner that thoseelements are alternately slipped in the vertical manner. This locationresults in dividing the opening portion by the storage capacitanceelement so that the opening portion is difficult to be visible, therebyavoiding the lowering of the resolution.

Next, the description will be directed to the arrangement of the storagecapacitance element which is capable of preferably preventing thediscontinuity of the display portion and the degradation of aresolution.

FIG. 32 shows an embodiment of the present invention. Each pixel isshown by a broken line of FIG. 32 in which only the opening portions(where the light is transmitted) 550, 551 are shown. As shown, eachpixel has two opening portions. The opening portions included in all thepixels are located at regular intervals. The width of the thin filmtransistor portion, which serves as the light-screening portion, issubstantially equivalent to the width of the storage capacitanceelement. The display is therefore divided by the narrow widths so as toprevent the lowering of the resolution.

The foregoing embodiments have described that the insulating film of thestorage capacitance element is a silicon nitride film or a compound filmcomposed of a silicon nitride film and an Al₂ O₃ film formed byanodizing Al. The insulating film of the storage capacitance element,however, employs an Al₂ O₃ film, a silicon nitride film, a compound filmcomposed of a silicon nitride film and an Al₂ O₃ film formed byanodizing Al, a Ta-anodized film, a compound film composed of a siliconnitride film and a Ta-anodized film, a compound film composed of asilicon nitride film and a Ta-anodized film, or a compound film composedof three or more layers. In other words, if the insulating film employsany film, the present invention keeps effective. Further, the commonelectrode line may be any compound film composed of Al, Ta, Cr or ITO ortwo metals of Ta, Cr and ITO. In particular, the compound film composedof ITO and another metal is advantageous in improving a numericalaperture, because the ITO is a transparent electrode. In case thepotential of the common electrode line is varied by the same frequencyas the signal potential, however, it is desirous that the resistance ofthe common electrode line is 2Ω or more per one pixel for preventing theuneven display resulting from the delay of a signal within thesubstrate. To design the storage capacitance element, however, it isnecessary to properly design the dimension of the connecting portion ofthe common electrode line in the consideration of the sheet resistanceof the common electrode line in order that the resistance is 2Ω or moreper one pixel.

As the size of one pixel is reduced as a result of making the displaymore definitive, the opening portion, that is, the display portion wherelight is transmitted is made smaller. Hence, it is necessary to takecare of the form of the opening portion. In general, when theorientation film is subject to rubbing for orienting the liquid crystalmolecules in the proper direction, the rubbing makes the display unevennear the step portion, resulting in often bringing about the orientationabnormal (domain). Hence, when the opening portion is made small, it isnecessary to take care of occurrence of the domain. The storagecapacitance element is located in a manner to allow the minimum width ofthe opening portion of the pixel to be 25 μm or more. In case theminimum width of the orientated portion of the pixel is 25 μm or more,the almost of the opening portion is formed at the location far off thestep portion. It results in allowing the even rubbing to be done,thereby eliminating the possibility of occurrence of an orientationabnormal.

Since the common electrode line is common to all the pixels within thedisplay panel, the pull-out portion of the end portion of the displaypanel is required to be designed so as to prevent the signal waveform ofthe common electrode line from being distorted. That is, in case theminimum width of the pull-out portion of the common electrode line islarger than the minimum width of the display panel, the distortion ofthe signal waveform is likely to take place. because the signal delayresulting from the wire resistance of the pull-out portion is small.

FIGS. 33A and 33B show the system to which the liquid crystal displayaccording to the invention is applied.

FIG. 33A shows the embodiment where the liquid crystal display isapplied to the display unit of the desk-top computer. As shown, thisembodiment includes a computer main unit 10, a keyboard 2, and a liquidcrystal display 3. In comparison with the conventional cathode-ray tube(referred to as CRT), the liquid crystal display according to theembodiment is advantageous in that it is lightweight and occupies lessarea. The feature of the liquid crystal display is more effective forthe system wherein two or more operators can work with one computer mainunit 10 by using two or more keyboards 2 and liquid crystal displays 3or the lap-top computer which requires a more lightweight display. Byapplying the liquid crystal display to the display portion of thecomputer, therefore, it is possible to realize a lightweight andspace-saving personal computer.

FIG. 33B shows another example to which the liquid crystal displayaccording to the invention is applied. In this example, the liquidcrystal display is used in part of a light shutter of the projectivetype display. The arrangement of the system includes a projecting unit 4having the liquid crystal display and an optical system, a screen 5, anda video signal processing unit (not shown). A video signal inputted fromthe external is converted into a proper signal format required fordisplaying an image on the liquid crystal display, for example, anon-interlaced RGB digital signal and then the image is displayed on theliquid crystal display depending on the converted signal format. Thedisplayed image is focused on the screen through the optical system. Ofthose components, the light shutter portion is a main factor fordefining the dimension of the optical system. The light shutter portionand thereby the overall optical system may be reduced in size by usingthe liquid crystal display where a lot of pixels are accommodated in asmall panel.

Further, since the liquid crystal display is lightweight and small insize, the use of the liquid crystal display results in realizing a colorsmall TV monitor or a large tapestry type TV.

As will be appreciated from the above description, the present inventionmakes it possible to reduce leakage of light resulting from the slippageof the upper and lower substrates from the proper positions and leakageof light directed in an oblique direction, resulting in implementing acolor display with a high contrast ratio. Since the numerical aperturecan be enlarged, the liquid crystal display can provide brighterdisplay. The invention can conserve the power consumption of thebacklight source, resulting in conserving the power consumption of theoverall device.

Moreover, the present invention also makes it possible to reduce thevariation of an optimal common potential on the plane and thereby thefluctuation of the optimal common potential and the threshold value. Theshort-circuit between the wires and the water mark are reduced,resulting in improving the yield. In addition, it is possible to preventthe discontinuity of the display portion, thereby allowing thedegradation of the resolution to be prevented.

What is claimed is:
 1. A liquid crystal display device for displaying animage comprising:a first substrate including a thin film transistor, aplurality of scan lines, a plurality of signal lines, and a plurality ofpixel electrodes connected to said thin film transistor, a secondsubstrate including a transparent common electrode, a liquid crystalwhich is sealed between said first and second substrates, a first opaquebody of a stripe-shape extending in a longer direction of said pixelelectrode formed on said first substrate, a second opaque body havingopening portion arranged in a matrix manner formed on said secondsubstrate, and wherein said first opaque body and said second opaquebody are disposed to overlap with each other.
 2. A liquid crystaldisplay device according to claim 1, wherein a storage capacitance lineis located on said first substrate between adjacent scan lines forcontrolling said thin film transistor and said first opaque body iselectrically separated from said storage capacitance line and said scanline and overlapped with a pixel electrode.
 3. A liquid crystal displaydevice according to claim 2, wherein a peripheral length of acapacitance portion of a storage capacitance element composed betweensaid storage capacitance line and a common electrode formed on thesecond substrate is set to be 1.33 or less times as large as a valueobtained by dividing an area of said capacitance portion by a size of adiagonal length of the area having said pixel electrodes locatedthereon.
 4. A liquid crystal display device according to claim 3,wherein the capacitance portion of said storage capacitance element iscomposed of an overlapped portion of two electrodes opposed to eachother and having different widths and lengths.
 5. A liquid crystaldisplay device according to claim 3, wherein said signal lines arelocated in a vertical direction, in parallel, and at regular intervals,said scan lines are located in a horizontal direction substantiallyperpendicular to said signal lines, in parallel, and at regularintervals, and said common electrodes are located in substantiallyparallel to said scan lines and in an alternate direction with said scanlines.
 6. A liquid crystal display device according to claim 3, whereina vertical width of said storage capacitance element is a value obtainedby dividing the area of the capacitance portion of said storagecapacitance element by a horizontal width of said common electrode andthe connecting portion between said common electrode and said storagecapacitance element is located on a substantially center in the verticaldirection of the vertical end portion of said storage capacitanceelement.
 7. A liquid crystal display device according to claim 3,wherein a vertical width of the portion where no light is transmitted inthe portion having said thin film transistor formed thereon issubstantially equivalent to a vertical width of a light-screeningportion of said storage capacitance element, said thin film transistorportion not serving as a display portion is separated from said storagecapacitance element, and the separation interval is substantiallyidentical for each pixel electrode.
 8. A liquid crystal display deviceaccording to claim 3, wherein a vertical end portion opposed to thecommon electrode composing one pixel electrode of said storagecapacitance element composes parallel lines and a horizontal end portionopposed to the transparent pixel electrode forming the other electrodeof said storage capacitance element also composes parallel lines.
 9. Aliquid crystal display device according to claim 3, wherein said storagecapacitance element is located on a substantially center of the pixelelectrode.
 10. A liquid crystal display device according to claim 1,wherein a storage capacitance line is located on said first substratebetween adjacent scan lines for controlling said thin film transistorand part of said storage capacitance line is overlapped with a pixelelectrode.
 11. A liquid crystal display device according to claim 1,wherein a corner area of the opening portion of the second opaque bodyformed on the second substrate is directed in an oblique manner or isformed to have a light-screening portion with two or more corners.
 12. Aliquid crystal display device according to claim 1, wherein theprojection of said first opaque body onto the second substrate isoverlapped with said second opaque body on an edge of said pixelelectrodes.
 13. A liquid crystal display device according to claim 1,wherein the width of the second opaque body is larger than an alignmentmargin of the first substrate and the second substrate in manufacturingthe liquid crystal display device.
 14. A liquid crystal display deviceaccording to claim 1, wherein either one of said first opaque body andsecond opaque body is divided into two or more portions.
 15. A liquidcrystal display device according to claim 1, whereinsaid first opaquebody is disposed along an edge of said opening portions of said secondopaque body.
 16. A liquid crystal display device according to claim 1,whereinsaid first opaque body is disposed so that a part of said firstopaque body overlaps with said pixel electrodes.
 17. A liquid crystaldisplay device according to claim 16, whereinsaid opening portions ofsaid second opaque body are disposed so that at least a part of saidopening portions is positioned inside of an overlapped region of saidfirst opaque body and said pixel electrodes.
 18. A liquid crystaldisplay device according to claim 17, further comprising:a light sourcedisposed at an opposite side of said first substrate with respect tosaid second substrate, wherein an edge of the opening portion of saidsecond opaque body is positioned spaced from an edge of said overlappedregion by at least a predetermined distance to prevent a light beam fromsaid light source from reaching the opening portion.
 19. A liquidcrystal display device according to claim 1, whereinsaid openingportions of said second opaque body have opposite edges expandingoutwardly to form convex-shaped portions at positions opposing to eachother.
 20. A liquid crystal display device according to claim 19,whereinsaid first opaque body is disposed along said convex-shapedportions of the edge of said opening portions.
 21. A liquid crystaldisplay device as claimed in claim 1, wherein said second opaque bodyhas said opening portions smaller than said pixel electrodes.
 22. Aliquid crystal display device according to claim 1, comprising a pair offirst constituent bodies constituting said first opaque body anddisposed corresponding to opening portions of second constituent bodiesconstituting said second opaque body,wherein widths of said openingportions are shorter than a distance between said pair of firstconstituent bodies.
 23. A liquid crystal display device according toclaim 1, further comprising a back light, wherein a width of the secondopaque body is smaller than a distance which an incident light incidentobliquely on said liquid crystal display device from said backlightpropagates in parallel with a surface of said first substrate or asurface of said second substrate when said incident light passes throughsaid liquid crystal.
 24. An information processing apparatus includingan information input means for inputting information, an informationprocessing means for processing said information, and a display meansfor displaying said information, said display means including a firstsubstrate having a thin film transistor, a plurality of scan lines, aplurality of signal lines, and a plurality of pixel electrodes connectedto said thin film transistor, a second substrate including a transparentcommon electrode, and a liquid crystal sealed between said firstsubstrate and said second substrate,a first opaque body of astripe-shape extending in a longer direction of said pixel electrodesformed on said first substrate, a second opaque body having openingportions arranged in a matrix manner formed on the second substrate, andwherein said first opaque body and said second opaque body are disposedto overlap each other.
 25. A projective type display using a liquidcrystal display device as a light shutter, said liquid crystal displaydevice comprising:a first substrate including a thin film transistor, aplurality of scan lines, a plurality of signal lines, and a plurality ofpixel electrodes connected to said thin film transistor, a secondsubstrate including a transparent common electrode, and a liquid crystalsealed between said first and second substrates, a first opaque body ofa stripe-shape extending in a longer direction of said pixel electrodesformed on said first substrate, a second opaque body having openingportions located in a matrix manner formed on said second substrate, andwherein said first opaque body and said second opaque body are disposedto overlap with each other.
 26. A liquid crystal display devicecomprising:a first substrate including a plurality of scan lines, aplurality of signal lines intersecting said scan lines, a thin filmtransistor formed at each intersecting portion of said scan lines andsaid signal lines, said thin film transistor being connected tocorresponding ones of said scan lines and said signal lines, and a pixelelectrode connected to said thin film transistor, a second substrateincluding a transparent common electrode, a liquid crystal sealedbetween said first and said second substrates, and a light sourcepositioned at an opposite side of said first substrate with respect tosaid second substrate, a first opaque body of a stripe-shaped layoutformed on said first substrate so that a part of said first opaque bodyoverlaps with said pixel electrode, a second opaque body having openingportions located in a matrix manner formed on said second substrate sothat said opening portions are positioned inside of an overlapped regionof said first opaque body and said pixel electrode, and an edge of saidopening portions is positioned spaced from an edge of the overlappedregion by at least a predetermined distance to prevent a light beam fromsaid light source from reaching the opening portion.
 27. A liquidcrystal display device including pixel electrodes controlled by activeelements being located in a matrix manner, comprising:a first substratehaving said active elements formed thereon, a second substrate opposedto said first substrate, a first opaque body of a stripe shape formed onsaid first substrate on an area other than an area on which said activeelements are formed, a second opaque body having opening portionslocated in a matrix manner, and formed on said second substrate, whereina projection of said first opaque body onto the second substrate isoverlapped with said second opaque body in the vicinity of an edge ofsaid pixel electrodes.
 28. A liquid crystal display device according toclaim 27, wherein either one of said first opaque body and said secondopaque body is divided into two or more portions.
 29. A liquid crystaldisplay device comprising:a first substrate including at least a thinfilm transistor, a plurality of scan lines, a plurality of signal lines,and a plurality of pixel electrodes connected to said thin filmtransistor, a first opaque body of stripe shape for screening lightapplied from a backlight source formed on an area other than an area onwhich said thin film transistor is formed, a second substrate includingat least a color filter and a transparent electrode, a liquid crystalsealed between both of said substrates, and a second opaque body havingopening portions located in a matrix manner on the second substrate,wherein a projection of said first opaque body onto the second substrateis overlapped with said second opaque body in the of an edge of thepixel electrodes.